The invention relates to a method and an apparatus for reducing the amount of sludges produced in a pulp and/or paper mill.
In pulp and paper mills different kinds of sludges and waste waters are produced in various process stages in connection with the manufacture of the end product. These include for example waste waters produced in the wood room and in chipping, waste waters produced in pulping, in cleaning processes of the produced pulp and in other treatments, as well as sludges and waste waters produced in connection with papermaking. At present, the aim is to recover as large an amount as possible of the fibers that have ended up in these sludges and waste waters and to recycle the fibers back to the process, such as papermaking. The remaining sludges and waste waters that contain for instance some amount of fibers, other lignin- and wood-containing particles, whitening chemical residues, coating agent particles used for coating and other waste waters are conveyed to waste water treatment.
In the treatment of waste water, sludges and waste waters are first treated in a chemi-mechanical preliminary sedimentation plant, in which so-called primary sludge is produced. Thereafter they are treated in biological waste water purification, which produces biosludge. After these treatments the remaining waste water can be discharged to a sewage network or conveyed back to the mill as process water.
The consistency of the primary sludge obtained from the mechanical preliminary sedimentation plant is typically 1 to 3%. The consistency of the biosludge obtained from an activated sludge plant and treated by a gravitation condenser is typically 2 to 3%. In order to be able to utilize these sludges or to dispose of them, water must be removed from them because their dry matter content is too low so that they could be treated as such. Typically mechanical water separators, such as band filter presses, clamps, and different kinds of centrifuges are used for this. By means of these the consistency of the sludge can be increased to 20 to 40%. Such sludge can already be conveyed to disposal or end use. Methods that are used quite frequently include for example taking the sludge to a waste disposal site, or composting the sludge. The sludge can also be dried and conveyed to soil improvement or it can be burned in a power boiler or waste heat boiler of the mill.
In the process of burning sludge, the sludge can be conveyed directly to the furnace of a power boiler, to be burned in addition to the fuel normally burned therein, such as bark, wood chips, sawdust or peat. The sludge can also be conveyed to be burned in the recovery boiler of the mill. In these waste disposal methods problems are still caused by the high moisture content of the sludge that impairs the efficiency of the boiler. The burning of sludge in the recovery boiler also requires a great deal of accuracy, because the large variations in the dry matter content of the sludge may cause a smelt explosion hazard.
It is also possible to dry the sludge thermally in a separate sludge drying device into dry matter content of 80 to 90%, and convey it as such to be burned. Thus, as thermal drying devices it is possible to use devices in which the sludge is brought in direct or indirect contact with hot air or gas having the temperature of over 100° C. One example of a drying device suitable for indirect drying of sludge is a rotary drier. This method requires that there is available in the mill a sufficiently large amount of primary gases i.e. hot air or gases with a sufficiently high temperature, necessary for drying the sludge.
One most frequently used method for disposing of the sludge is to burn it in such a manner that the sludge is mixed with the solid fuel used in the boiler, such as bark or wood chips, before the bark or wood chips are conveyed to the furnace. It is also possible to dry the mixture of sludge and bark or wood chips further before the mixture is conveyed to burning, as disclosed in the WO patent publication 96/21624.
The above-mentioned methods for reducing the amount of sludges produced in pulp and/or paper mills by burning them mixed into the fuel of the power boiler do work as such and reduce the amount of sludges of the mill to be taken to the waste disposal site. As was mentioned above, problems are caused by the water contained in the sludge, which impairs the efficiency of the power boiler burning the sludge. Tightening environmental requirements that pose restrictions for the amount of sludge to be placed in the waste disposal site result in that it is necessary to dispose of a larger amount of waste by means of other methods.